Radon is a colorless, odorless, and tasteless radioactive gas that occurs naturally in the environment. This gas is an environmental concern because it can accumulate to dangerous levels inside buildings, posing a serious threat to air quality. Understanding the source and behavior of this substance is necessary to properly categorize it within environmental science.
Defining Pollutant Categories
Air pollutants are broadly classified into two major categories based on their method of entry into the atmosphere. Primary pollutants are substances that are emitted directly from an identifiable source in their final, harmful form. Examples include carbon monoxide from car exhaust or sulfur dioxide released from industrial smokestacks.
Secondary pollutants are not emitted directly but instead form within the atmosphere through chemical reactions. These reactions usually involve primary pollutants reacting with other atmospheric components, often in the presence of sunlight. Ground-level ozone, a major component of smog, is a common example of a secondary pollutant.
How Radon Forms and Enters the Air
Radon gas originates from the natural radioactive decay chain of heavier elements found in nearly all soil and rock worldwide. Specifically, it is a product of the decay of uranium-238, which transforms into radium-226 before decaying into the gaseous radon-222 isotope. The continuous breakdown of uranium in the earth’s crust constantly produces radon, which is then released into the surrounding soil and air.
As a gas, radon moves up through the ground and can enter buildings through any opening that contacts the soil. Common entry points include cracks in the foundation, construction joints, gaps around utility pipes, and sumps. The air pressure inside a building is often slightly lower than the pressure in the soil beneath it, which draws the radon gas indoors, allowing it to accumulate. The source of the gas is entirely geological and occurs without any human-caused chemical reaction.
The Classification of Radon
Radon is classified as a primary pollutant because it is released into the environment in its final, chemically unchanged state. The gas is generated underground through natural radioactive decay and is emitted directly from the soil into the air. It does not require a reaction with sunlight or other atmospheric chemicals to become a hazardous substance.
Radon fits the definition of a primary pollutant perfectly, despite its natural origin, as it is emitted directly from an environmental source. Its fundamental characteristic is that it enters the air directly, bypassing the atmospheric synthesis that defines a secondary pollutant.
Health Risks Associated with Radon Exposure
The classification of radon is important because of its significant danger to human health, particularly when it becomes concentrated indoors. Once inhaled, the radon gas itself is not the primary threat, but its short-lived decay products, known as progeny, attach to dust particles and are deposited in the lungs. These decay products are solid radioactive particles that continue to break down, releasing energy.
The resulting release of alpha particles, a form of ionizing radiation, can damage the DNA of lung tissue, potentially leading to the development of cancer. Radon is the second leading cause of lung cancer overall, following smoking, and is considered the leading cause among people who have never smoked. The long-term exposure to this hazard, especially in enclosed spaces, causes an estimated 21,000 lung cancer deaths annually in the United States. Because of this serious health risk, testing homes for radon levels and installing mitigation systems to vent the gas away from the living space are necessary public health actions.